Rockland County Water Use

Different ways of measuring the amount of water used serve different purposes. The highest daily use in a year, or peak demand, is important in understanding requirements for fire protection and increased seasonal demand. It's only necessary to meet peak demand for a short period of time, however, and average demand, measured in millions of gallons/day (mgd) is used to assess the adequacy of a water provider's sustainable water supply. In assessing the potential role of increased water conservation in long-term water supply plan it's useful to look at per capita usage. We take all three perspectives below. Click here for revised April 25, 2010 presentation on Rockland County Water Use and Conservation Potential (2/16/11). Click here for April 2, 2013 public talk at Rockland Community College on Water Conservation Opportunities in Rockland County.

Annual Average Demand (MGD)mgd = million gallons per day

1970

19.7

1971

19.9

1972

20.8

1973

21.8

1974

22.5

1975

23.2

1976

24.2

1977

25.6

1978

25.2

1979

25.3

1980

25.9

1981

22.4

1982

23.1

1983

25.4

1984

26.1

1985

24.3

1986

24.6

1987

26.6

1988

27.7

1989

26.7

1990

27.4

1991

29.9

1992

28.3

1993

28.8

1994

28.6

1995

28.4

1996

28.0

1997

27.5

1998

28.9

1999

28.8

2000

28.6

2001

29.7

2002

26.7

2003

28.5

2004

29.2

2005

31.7

Maximum Day Demand (MGD)mgd = million gallons per day

1970

29.8

1971

33.0

1972

28.6

1973

36.2

1974

37.9

1975

35.7

1976

33.4

1977

43.0

1978

37.5

1979

37.1

1980

40.8

1981

30.7

1982

31.4

1983

37.0

1984

37.3

1985

27.7

1986

34.4

1987

38.9

1988

41.6

1989

33.1

1990

34.2

1991

44.9

1992

35.7

1993

40.7

1994

38.6

1995

40.0

1996

36.8

1997

39.2

1998

42.9

1999

44.6

2000

39.1

2001

46.5

2002

32.1

2003

37.4

2004

40.3

2005

43.5

Source: Dr. Daniel Miller, Rockland County Hydrologist, testimony to the Public Service Commission, 2006.
Minor typos in original testimony corrected here.

KEY

Drought. Not included in max demand analysis because max demand artificially reduced. Not representative.

Drought. However, included in analysis since water use restrictions were declared in Sept, well after max-day demand occurred.

Historical Average Demand in Rockland County

United Water data from 1970-2005 show a general increase in average demand from the low of 19.7mgd in 1970 to the high of 31.7mgd in 2005. To put this in perspective, Rockland County sustainable water supply was assessed by UWNY to be 32-34 mgd in 2006, barely above annual demand in 2005.

Historical Peak Demand in Rockland County

United Water data from 1970-2005 show a general increase from 29.8mgd in 1970 to 43.5mgd in 2005. The low was 27.7mgd in 1985 and the highest daily demand in the period was 46.5mgd in 2001. To put this in perspective, Rockland County peak day capacity is from 44.5 - 47.5mgd. In other words, Rockland County peak demand in the summer is approaching peak day capacity. Peak day capacity is defined as the maximum amount of water that can be delivered in a day, given the currently developed resources (wells and surface waters).

In looking at the historical peak demand it's helpful to know that requests for voluntary conservation were made in 1981, 1982 and 1985 before mandatory drought restrictions were enacted as part of the Rockland Sanitary Code, and mandatory water restrictions were invoked in 1995, 1999 and 2002.

Rockland County Seasonal Water Usage

updated 2/16/2011

Single-family residences are the largest group of water users in Rockland County, with average daily per capita usage between 2000 and 2009 ranging from 62.49 gallons per capita per day (gpcd) in 2009 to 72.89 in 2001. Looking at residential usage data on a per capita basis provides us with a way of assessing conservation potential in the future. In the table to the right, estimated indoor usage is an average of daily per capita usage from mid-October through mid-April. Indoor and outdoor use is an average of daily per capita usage for mid-April through mid-October. The estimated outdoor use is the difference between the two.

Compared to the study locations in the Residential Water Usage Study cited below, Rockland County's average increased summer usage is relatively low during this period at 11.54 gpcd. Rockland County's average non-summer usage at 62.15 gpcd is roughly 10% below the national indoor usage average of 69 gpcd. Comparing this number to the standard for an EPA WaterSense conserving home of 40 gpcd suggests indoor water conservation measures could be an important component in Rockland County's water supply planning going forward. In order to meet peak demand requirements, however, the peak daily demand in relationship to average daily demand is critical, and that ratio in Rockland County is approximately 1.6.

Single family residential per capita use by year (gpcd)

Year

Average daily use

Average estimated indoor use

Summer indoor + outdoor use

Estimated average outdoor use

2000

69.21

64.29

73.71

9.42

2001

72.89

64.95

80.72

15.77

2002

64.84

63.06

66.57

3.51

2003

66.49

61.72

71.21

9.49

2004

66.70

61.91

71.10

9.19

2005

71.36

62.14

80.45

18.31

2006

69.37

62.29

76.30

14.01

2007

69.90

60.50

79.12

18.62

2008

66.96

60.39

73.05

12.66

2009

62.49

60.24

64.67

4.43

Average 2000-2009

68.02

62.15

73.69

11.54

gcpd = gallons per capita per day

How does the new (2/2011) set of calculations for single-family residential water usage differ from the old calculations?

The people/ household factor used to convert single-family residential accounts to people was changed from 3.0 to 3.24. The old factor was an average of single-fanily residential, multi-family and apartment while the new factor represents single-family residential only. In addition the assumption about which months represent indoor use only was changed in order to match UWNY assumptions for easier discussion and comparison.

Will the water usage calculations and indoor/outdoor estimates change again?

We expect to refine the indoor/outdoor estimates in the future as more data becomes available and we are beginning to look at the development of weather-adjusted usage estimates.

Future Water Use

In order to determine requirements for future water supply both the Rockland County Bureau of Water Supply and United Water of New York estimate what demand for water will be in the future. They look at both peak day demand (the highest water use day of the year) and average water use (average daily use for a year).

Peak Demand

Rockland County's approach to projecting future demand was to start first with a straight line regression, predicting essentially that water use in the county will continue to increase in the future at the same rate that it has increased in the past. When making this estimate Rockland County removed the years when demand was lower than normal due to mandatory conservation restrictions during declared droughts. In this way they estimate the future growth of unconstrained demand during non-drought years, resulting in the following peak day demand projections:

2010 - 43.5MGD
2015 - 44.7MGD
2020 - 45.9MGD

It turns out however that because of the considerable year-to-year variability the actual maximum day demand was greater than predicted by linear regression 50% of the time etween 1970 and 2005. As a result, when Rockland County projects future demand for water it identifies a range so that it can estimate a maximum day demand number that will be as high or higher than the actual number 80, 90, or 99% of the time.

By excluding drought years, these projections account for existing regulations (Article V of the Sanitary Code) that can be used to reduce demand during drought.

UWNY uses a different methodology to estimate future water usage with numbers that are generally higher than the Rockland County projections but generally within the 99% confidence interval. UWNY projections show an increased rate of increase in water usage out to 2020, in contrast to the Rockland County projections that predict a stable rate of increase. In his testimony to the Public Service Commission the Rockland County hydrologist speculated that the projected increased rate of increase is "likely due to the style of development recently experienced in Rockland, i.e., large single-family homes with extensive landscaping and irrigation systems." (Miller, 2006)

Average Demand

For average demand Rockland County and UWNY take the same approaches they did with peak demand with two differences:

1. Rockland County began its regression to estimate future average demand in 1981 rather than 1970 because water conserving plumbing fixtures were introduced in 1980 and the rate of increase in average water use significantly slowed.

2. UWNY's average demand projections are lower than Rockland County's 80% confidence level, suggesting that 10% of the time actual demand will be exceeded.

New York State Laws and Regulations
NYS Regulations 3.1.1 Recommended Standards for Water Works, 2003 Edition, incorporated by reference in 10 NYCRR, Part 5, Subpart 5-1.222 require that.

1. For a surface water supply the quantity of water at the source shall be adequate to meet the maximum projected water demand of the service area as shown by calculations based on a one in fifty year drought or the extreme drought of record and should include considerations of multiple year droughts and provide a reasonable surplus for anticipated growth.

2. Total developed groundwater source capacity, unless otherwise specified by the reviewing authority shall equal or exceed maximum day demand with the largest producing well out of service.